I talked with a good friend of mine who is in programming. He asked me if the hairstyles have "altered models" for different types of hats. Apparently, in order to make hats work for Viera they would need to create a few altered models that would fit around the different types of headgear, for every hairstyle. For example: with the general hairstyles when they have, let's say the Racing Chocobo Mask equipped, all the portion of the hairstyle that is "under" the helm is actually "deleted", this is to ensure those faces of the mesh don't get counted in the rendering process, making the game do less calculations and ensuring it runs smoothly, 1.0 flower vase anyone?
Now, this is very doable, but it would take time and resources to be put into making the few altered models for each hairstyle and refitting them for all hats in the game. But the problem is all that work would be for only 1 race, so the devs might see that as the last task on their priority list. To make it work for Hrothgar, you do all that + refit it around their heads.
It is not so much as hard or impossible, but more of a: we do not have the time, resources or urgency to be spending on this right now. So we will put it on the back burner for as long as we can.
Now take what I said with a grain of salt. Neither my friend nor I work for SE, nor do we know the ins and outs of their working schedule. This is just our guess based on our work experience.
